PHYSICS FOR ECONOMY vol. 1/2018, pp. 55-68 Andrey VARLAMOV 1 Zheng ZHOU 2 Yan CHEN 3 BOILING, STEAMING OR RINSING? (THE PHYSICS OF CHINESE CUISINE) Some physical aspects of Chinese cuisine are discussed. We start from the cultural and historical particularities of Chinese cuisine and their food production technolo- gies. What is the difference between raw and boiled meat? What is the difference in the physical processes of heat transfer during either the steaming of dumplings or during cooking them in boiling water? Why is it possible to cook meat stripes in a "hot pot" in ten seconds, whilst baking a turkey requires several hours? This article is devoted to a discussion of these questions. Keywords: complex organic molecules, proteins, damplings, heat transfer, temper- ature conductivity, Fourier law, Newton’s law of cooling Globalization, which is rapidly taking place throughout the world, is vividly manifested by the ubiquitous availability of dishes of various cuisines from all over the world. Of course, as a rule, these have only some semblance of the true masterpieces of culinary art: in addition to the skill of the cook, the creation of the latter requires the corresponding products. As our familiar Italian gastronomic critic, Sergio Grasso, says, "food does not go to a person, this person should travel to food." Chinese cuisine is one of the richest and most interesting cuisines in the world. Here everything, or, well, almost everything, can be eaten. And the ways of cooking are very different. One of the authors (AV) had a chance during a visit to Shanghai to open a small door into this wide world and, under the guidance of the two other authors, make some first steps in it, being interested not only in the exotic tastes but also in the underlying unusual physical processes. Speaking about Chinese cuisine, the first things that come to mind, probably, are the dumplings, worldwide the most popular Chinese dish, which you can eat in Chicago, Canberra, or Moscow. Dumplings are honoured all over China, especially in Jiangnan region of China (close to the Yangtzi Delta). Three types of dumplings are commonly seen, especially in Shanghai and Suzhou, namely, 1 Andrey Varlamov, CNR-SPIN, Viale del Politecnico 1, I-00133 Rome, Italy 2 Zheng Zhou, Department of Physics, Fudan University, Shanghai, China 3 Yan Chen, Department of Physics, Fudan University, Shanghai, China 56 A. Varlamowv, Z. Zhou, Y. Chen Siaulon Pau , Santsie Moedeu and Wonton . Made of a thin dough, filled with pork, the fat of which melts into the soup when cooked, and served with Chinkiang vinegar, they are all pretty much the same. The most significant difference is the method of their heat treatment. Santsie Moedeu (large size dumpings) are pan- fried, this process is called in Shanghai “Santsie ”. Wonton dumplings are boiled in 100 °C, whereas Siaulon dumplings are processed also at 100°C, but in an atmosphere of saturated steam in small bamboo steaming baskets which are called "Siaulon" (see Fig. 1). Fig. 1. The three kinds of dumplings in Jiangnan, China. The left one is Santsie Moedeu, the middle one is Siaulon Pau and the right one is Wonton Hotpot is also a popular type of Chinese cuisine. Originating in Mongolia more than 1,000 years ago and gaining its popularity in the times of the Qing Dynasty 4 all over the country, hotpot boasts a profound history. During its spread, hotpot was diversified into many variations. Beijing hotpot lays particular emphasis on the soup base and sauces, Chongqing hotpot boasts a stimulating and refreshing “ Ma La ” (“ 麻辣 ”, “numb and spicy”) flavor, and Chaoshan hotpot is famous for its deliberately-prepared thin-cut mutton, named “ Shuan Yangrou ” (“ 涮羊肉 ”) in Chinese. When enjoying hotpot, one puts ingredients such as beef balls, fish balls, crab meat, or vegetable slices into the elaborately prepared soup base and waits for it to be done. After picking it up and dipping it in the sauce, delicious food is ready to eat. The whole process is called “ Zhu” (“ 煮”, “to boil”) in Chinese, and takes 5-10 minutes or so, when applied to meat balls or vegetables. Remarkably, in the meantime, another process can be used to get a different kind of food in the same hotpot, but much quicker. In Chinese it is called “Shuan” (“ 涮”, “rinse” or “instant-boil”). It consists of soaking the thin-cut sliced beef or mutton in the boiling soup. Surprisingly in only 10 seconds the sliced beef changes it colour 4 The Qing dynasty was the last imperial dynasty of China, established in 1636 and ruling China from 1644 to 1912 with a brief, abortive restoration in 1917. It was preceded by the Ming dynasty and succeeded by the Republic of China. Boiling, steaming or rinsing? … 57 from pink to white or gray, indicating that the slice is ready to eat. The beef slice becomes ready even without being let go of by the chopsticks (see Fig. 2). Fig. 2. Cooking in the hotpot Today, cooking has become not only a giant industry, not only an art, but also a vast field of science. Here, biology, chemistry, physics, economics, ethics and many more intersect. The tasks of this science are infinite. All the time, new methods of cooking appear. We will not even try to list them here – neither those for the frying of meat, nor those for baking turkey, nor even the preparation of a BBQ on charcoal, none of them will be discussed here. Instead let's talk about the physical processes underlying cooking in the examples of the dishes described above - about the physics of boiling, steaming, and “rinsing in hotpot”. Boiling What is the essence of the process of boiling meat? In everyday language, raw meat should become cooked. And what does this mean "scientifically"? Fig. 3. Protein denaturation Meat, basically, consists of complex organic macromolecules called proteins (the type of protein varies depending on the type of meat). In raw meat, the protein molecules are in a state of long entangled chains (Fig. 3). In the course of heat treatment, the temperature rises and these chains straighten, and when the tem- 58 A. Varlamowv, Z. Zhou, Y. Chen perature reaches value T d, specific for each type of meat, they are compactified into a kind of "carpet". This process is called protein denaturation. It occurs at relatively low temperatures: for meat it is 55~80°C, and for fish the temperature is even lower. In any case, anyone who has ever eaten a chicken soup can be sure that boiling it at 100°C turns out to be sufficient for completely compacting the proteins in the meat. From the point of view of physics, the states of proteins in raw meat and boiled meat differ in their energy. To turn the protein from its native state to the denaturated one, an energy barrier must be overcome (Fig. 4). At room temperature, this barrier is high. In the process of cooking, the temperature rises. Correspondingly the energy of the protein changes, as is shown in Fig. 4. Having reached the top of the "hill", the protein falls down to the new state – a denatured protein – the meat is cooked! This is what happens in a pot of boiling soup. 5 Fig. 4. A schematic presentation of the process how protein overcomes the energy barrier when the temperature increases So, the first task of the cook when boiling meat in terms of physics is to increase the temperature throughout the volume of the piece to at least the tem- perature of denaturation. 5 It should be noted that in recent years became fashionable to cook meat at relatively low temper- atures, the so-called "sous-vide" method. The meat is placed in a thermostat with a temperature somewhat lower than that of denaturation. Each separate macromolecule lack energy alone to jump over the barrier. However, it can occasionally "borrow" it from the environment. So, grad- ually (it takes a long time - many hours, maybe even a day), all bulk of the meat transfers into the denaturated state. Boiling, steaming or rinsing? … 59 Fig. 5. Penetration of heat in the spherically symmetrical piece of meat In the light of the above, we formulate the simplest model of the process of meat cooking. Let a spherically symmetric homogeneous piece of meat (radius R) with an initial temperature T0 and a thermal conductivity coefficient κ be placed in an environment where a fixed temperature Te is maintained. How much time does it take for the temperature of the meat in the center of the ball to reach Tg? (Fig. 5) In mathematical physics, the process of heat transfer inside a sphere is de- scribed by a complicated differential equation [1] (1) where T(r, t) is the temperature at a point r at time t, κ is the thermal conductivity of the meat, ρ is its density, and c is the specific heat. Since the water is boiling in a saucepan, the temperature at the surface of the sphere at any instant of time remains constant and equal to 100°C (2) We took the meat from the refrigerator, so at the time when it was dropped into the water, the temperature was 4°C throughout its volume: (3) Equations (1)-(3) determine the problem of a differential equation with boundary conditions. How to deal with them is well known for mathematicians, and knowing the numerical values of the thermal conductivity of meat, its density and specific heat, they will be able to accurately write a recipe for cooking broth. 60 A.